Step stool



FIG. 1 is a top right front perspective view of a step stool showing our new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a right side elevational view thereof, the left side elevational view being a mirror image;

FIG. 4 is a front elevational view thereof; the rear elevational view being a mirror image; and,

FIG. 5 is a bottom plan view thereof.

The broken line showings of circular tread in FIGS. 1 and 2 are for environmental purposes only and form no part of the claimed design. 

The ornamental design for a step stool, as shown and described. 